Messages in this thread | | | Subject | Re: [PATCH v1 3/4] arm64: Fix the page leak in pud/pmd_set_huge | From | Chintan Pandya <> | Date | Wed, 14 Mar 2018 16:57:29 +0530 |
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On 3/14/2018 4:23 PM, Mark Rutland wrote: > On Wed, Mar 14, 2018 at 02:18:24PM +0530, Chintan Pandya wrote: >> While setting huge page, we need to take care of >> previously existing next level mapping. Since, >> we are going to overrite previous mapping, the >> only reference to next level page table will get >> lost and the next level page table will be zombie, >> occupying space forever. So, free it before >> overriding. > >> @@ -939,6 +940,9 @@ int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot) >> return 0; >> >> BUG_ON(phys & ~PUD_MASK); >> + if (pud_val(*pud) && !pud_huge(*pud)) >> + free_page((unsigned long)__va(pud_val(*pud))); >> + >> set_pud(pudp, pfn_pud(__phys_to_pfn(phys), sect_prot)); >> return 1; >> } >> @@ -953,6 +957,9 @@ int pmd_set_huge(pmd_t *pmdp, phys_addr_t phys, pgprot_t prot) >> return 0; >> >> BUG_ON(phys & ~PMD_MASK); >> + if (pmd_val(*pmd) && !pmd_huge(*pmd)) >> + free_page((unsigned long)__va(pmd_val(*pmd))); >> + > > As Marc noted, (assuming the subsequent revert is applied) in both of > these cases, these tables are still live, and thus it is not safe to > free them. > > Consider that immediately after freeing the pages, they may get > re-allocated elsewhere, where they may be modified. If this happens > before TLB invalidation, page table walks may allocate junk into TLBs, > resulting in a number of problems. Ah okay. What about this sequence, 1) I store old PMD/PUD values 2) Update the PMD/PUD with section mapping 3) Invalidate TLB 4) Then free the *leaked* page
> > It is *never* safe to free a live page table, therefore NAK to this > patch. > > Thanks, > Mark. >
Chintan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
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